Gas discharge lamps are commonly known, and an elaborate description of the design of such lamps is therefore not necessary in this document. A gas discharge lamp comprises two electrodes located in a closed vessel filled with an ionizable gas or vapor. The vessel is typically quartz or a ceramic material, for instance, polycrystalline alumina (PCA). The electrodes are arranged at a certain distance from each other, and an electric arc is maintained between these electrodes during operation.
A gas discharge lamp may be powered by an electronic driver. Electronic drivers are commonly known, and an elaborate description of the design of such electronic drivers is therefore not necessary in this document. The driver has two important tasks. One task is maintaining a gas discharge during steady-state operation; in a typical design, the driver produces a commutating current applied to the lamp at a commutation frequency of the order of about 100 Hz. However, in principle, direct current operation is also possible. In other design variants, lower or higher commutation frequencies are also possible. Another task of the driver is igniting the lamp when it is off. Typically, this is achieved by applying, in an ignition phase, a voltage signal at a relatively high voltage. In principle, this may be a constant voltage, but better results are obtained by applying voltage pulses or a resonant voltage.
The present invention particularly relates to an electronic driver having an ignition functionality on the basis of a resonant voltage.
An important problem related to ignition is that the voltage required for ignition depends on the condition of the lamp. Particularly when the lamp has just extinguished and is still hot, the voltage required for re-ignition may be much higher as compared to the situation when the lamp is cold. For instance, the ratio between the required re-ignition voltage in a hot condition and this voltage in a cold condition may be at least 2, and even 7 or 8. To ensure that the lamp ignites whatever its condition, it should therefore be possible to always apply the high voltage required for hot re-ignition. However, this high voltage, which may be as high as 5 to 8 kV in favorable situations, is still so high that it may reduce the lifetime of the lamp if applied without reason. Furthermore, in the case of a broken lamp, components in the path from the driver to the lamp get damaged by the high voltage. It is therefore desirable to apply a lower ignition voltage for cold ignition.
One possible way of overcoming this problem is to use two igniters: one for applying a relatively “low” high voltage when the lamp is cold, and another for applying a “higher” high voltage when the lamp is hot. However, such a solution is not practical and is expensive.